Birds build nests for their young. The female bird lays eggs in the nest, and when the eggs hatch, the baby birds remain in the nest until they are strong enough to fly off on their own. There were two nests in my backyard last summer. Each one had baby birds in it:

You can use the word "nest" for things other than as a home for birds:

There's a hornets' nest in the garage. Be careful or they'll sting you!

My son's bedroom looks like a rat's nest. It's very messy.

Bob and Kathy's children have all left the nest. (The word "nest" in this sentence serves as a metaphor for home.)

Bob and Kathy are empty-nesters. (This means that their children are all adults and have left home.)